Fallout – Is it possible to have a Power Armor without the helmet

The quality of items you find is directly tied to the length of time the dweller is out in the wasteland. Their survivability of course is related to their level (health) and the number of stims/radaways you give them.

Basically just wait longer. The longer they are out there, the better chance of getting a better drop. There are no guarantees of course, but if they can survive out there for several more days, leave them for as long as you can and you should get better drops. Since they take no damage or radiation while on their way back, you can wait until they are literally 1 hp away from death before recalling them to get maximum exploration time.

Yes it is possible, I have found Legendary outfits and weapons while exploring the wasteland. It is definitely easier to find gold gear in lunch boxes, but it is still possible to get really lucky while wandering.

To be specific, I found a "Heavy Combat Armor" (4S 3E) in one vault and an "Tuned Alien Blaster" (18-20 dmg) in a different vault.